Removal of Surface Oxides by Electron Attachment

ABSTRACT

The present invention relates to a method for removing metal oxides from a substrate surface. In one particular embodiment, the method comprises: providing a substrate, a first, and a second electrode that reside within a target area; passing a gas mixture comprising a reducing gas through the target area; supplying an amount of energy to the first and/or the second electrode to generate electrons within the target area wherein at least a portion of the electrons attach to a portion of the reducing gas and form a negatively charged reducing gas; and contacting the substrate with the negatively charged reducing gas to reduce the metal oxides on the surface of the substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in part of U.S. patent applicationSer. No. 12/042,055, which, in turn, is a continuation of U.S. patentapplication Ser. No. 10/425,405, filed Apr. 28, 2003, the disclosures ofwhich are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates generally to fluxless processes forremoving metal oxides from the surface of a substrate, including aninsulated substrate.

Wafer bumping is a process used to make thick metal bumps on the chipbond pads for inner lead bonding. The bumps are commonly made bydepositing a solder on the pads and then reflowing (referred to hereinas a first reflow) to conduct alloying and to change the shape of thesolder bump from a mushroom-shape into a hemispherical-shape. The chipwith the first-reflowed bumps is “flipped” to correspond to thefootprint of the solder wettable terminals on the substrate and thensubjected to a second reflow to form solder joints. These solder jointsare referred to herein as inner lead bonds. High-melting point solders(e.g., >300° C.) are normally used in the wafer bumping process becauseit allows for subsequent assembly steps such as outer lead bonding toproceed using lower-melting point solders (e.g., <230° C.) withoutdisruption of the inner lead bonds.

The shape of the solder bumps after the first reflow is critical. Forexample, a large bump height is preferable for better bonding and higherfatigue resistance. Further, the bumps formed should preferably besubstantially uniform to ensure planarity. Substantially uniform solderbumps having relatively larger bump heights is believed to be associatedwith an oxide-free bump surface during the first reflow. Currently,there are two major approaches to removing solder oxides during thefirst reflow of the solder bumped wafer. One approach is fluxlesssoldering using pure hydrogen at a reflow temperature of 400 to 450° C.The major challenge of this approach is the flammable nature of the purehydrogen, which largely limits the application of this approach. Thesecond approach is applying organic fluxes over the deposited solderbumps, or within a solder paste mixture that has been printed onto thewafer to form the bumps, and reflowing the bumps in an inert environmentso that the fluxes can effectively remove initial oxides on the soldersurface. However, this approach has its drawbacks. Small voids may formin the solder bumps due to flux decomposition. These voids may not onlydegrade the electrical and mechanical properties of the formed solderbonds but also destroy the co-planarity of the solder bumped wafer andaffect the subsequent chip bonding process. The decomposed fluxvolatiles can also contaminant the reflow furnace which can increase themaintenance cost. In addition, flux residues are oftentimes left uponthe wafer which can corrode metals and degrade the performance of theassembly.

To remove the flux residues from the reflow processes described above, apost cleaning process may be adopted using chlorofluorcarbons (CFCs) ascleaning agents. However, post-cleaning adds an additional process stepand increases the manufacturing processing time. Further, the use ofchlorofluorocarbons (CFCs) as cleaning agents is banned due to thepotential damage to the earth's protective ozone layer. Althoughno-clean fluxes have been developed by using a small amount ofactivators to reduce residues, there is a trade-off between the gain andloss in the amount of flux residues and the activity of the fluxes.Therefore, a catalytic method to assist generating highly reactive H₂radicals, and thus reducing the effective ranges of hydrogenconcentration and processing temperature for reducing surface oxides,has been sought by the industry.

Fluxless (dry) soldering has been performed in the prior art usingseveral techniques. One technique is to employ lasers to ablate or heatmetal oxides to their vaporization temperatures. Such processes aretypically performed under inert or reducing atmospheres to preventre-oxidation by the released contaminants. However, the melting orboiling points of the oxide and base metal can be similar and it may notbe desirable to melt or vaporize the base metal. Therefore, such laserprocesses are difficult to implement. Lasers are typically expensive andinefficient to operate and require a direct line of sight to the oxidelayer. These factors limit the usefulness of laser techniques for mostsoldering applications.

Surface oxides can be chemically reduced (e.g., to H₂O) through exposureto reactive gases (e.g., H₂) at elevated temperatures. A mixturecontaining 5% or greater reducing gas in an inert carrier (e.g., N₂) istypically used. The reaction products (e.g., H₂O) are then released fromthe surface by desorption at the elevated temperature and carried awayin the gas flow field. Typical process temperatures exceed 350° C.However, this process can be slow and ineffective, even at elevatedtemperatures.

The speed and effectiveness of the reduction process can be increasedusing more active reducing species. Such active species can be producedusing conventional plasma techniques. Gas plasmas at audio, radio, ormicrowave frequencies can be used to produce reactive radicals forsurface de-oxidation. In such processes, high intensity electromagneticradiation is used to ionize and dissociate H₂, O₂, SF₆, or otherspecies, including fluorine-containing compounds, into highly reactiveradicals. Surface treatment can be performed at temperatures below 300°C. However, in order to obtain optimum conditions for plasma formation,such processes are typically performed under vacuum conditions. Vacuumoperations require expensive equipment and must be performed as a slow,batch process rather than a faster, continuous process. Also, plasmasare typically dispersed diffusely within the process chamber and aredifficult to direct at a specific substrate area. Therefore, thereactive species cannot be efficiently utilized in the process. Plasmascan also cause damage to process chambers through a sputtering process,and can produce an accumulation of space charge on dielectric surfaces,leading to possible microcircuit damage. Microwaves themselves can alsocause microcircuit damage, and substrate temperature may be difficult tocontrol during treatment. Plasmas can also release potentially dangerousultraviolet light. Such processes also require expensive electricalequipment and consume considerable power, thereby reducing their overallcost effectiveness.

U.S. Pat. No. 5,409,543 discloses a process for producing a reactivehydrogen species (i.e., atomic hydrogen) using a hot filament tothermally dissociate molecular hydrogen in a vacuum condition. Theenergized hydrogen chemically reduces the substrate surface. Thetemperature of the hot filament may range from 500° C. to 2200° C.Electrically biased grids are used to deflect or capture excess freeelectrons emitted from the hot filament. The reactive species or atomichydrogen is produced from mixtures containing 2% to 100% hydrogen in aninert carrier gas.

U.S. Pat. No. 6,203,637 discloses a process for activating hydrogenusing the discharge from a thermionic cathode. Electrons emitted fromthe thermionic cathode create a gas phase discharge, which generatesactive species. The emission process is performed in a separate orremote chamber containing a heated filament. Ions and activated neutralsflow into the treatment chamber to chemically reduce the oxidized metalsurface. However, such hot cathode processes require vacuum conditionsfor optimum effectiveness and filament life. Vacuum operations requireexpensive equipment, which must be incorporated into soldering conveyorbelt systems, thereby reducing their overall cost effectiveness.

Potier, et al., “Fluxless Soldering Under Activated Atmosphere atAmbient Pressure”, Surface Mount International Conference, 1995, SanJose, Calif., and U.S. Pat. Nos. 6,146,503, 6,089,445, 6,021,940,6,007,637, 5,941,448, 5,858,312 and 5,722,581 describe processes forproducing activated H₂ (or other reducing gases, such as CH₄ or NH₃)using electrical discharge. The reducing gas is generally present at“percent levels” in an inert carrier gas (N₂). The discharge is producedusing an alternating voltage source of “several kilovolts”. Electronsemitted from electrodes in a remote chamber produce exited or unstablespecies that are substantially free of electrically charged species,which are then flowed to the substrate. The resulting processes reduceoxides on the base metal to be soldered at temperatures near 150° C.However, such remote discharge chambers require significant equipmentcosts and are not easily retrofitted to existing soldering conveyor beltsystems. In addition, these processes are typically employed forpre-treating the metal surface before soldering rather than removingsolder oxides.

U.S. Pat. No. 5,433,820 describes a surface treatment process usingelectrical discharge or plasma at atmospheric pressure from a highvoltage (1 kV to 50 kV) electrode. The electrode is placed in theproximity of the substrate rather than in a remote chamber. The freeelectrons emitted from the electrodes produce reactive hydrogenradicals—a plasma containing atomic hydrogen—which then pass throughopenings in a dielectric shield placed over the oxidized substrate. Thedielectric shield concentrates the active hydrogen onto those specificsurface locations requiring de-oxidation. However, such dielectricshields can accumulate surface charge that may alter the electric fieldand inhibit precise process control. The described process is only usedto flux base metal surfaces.

Accordingly, there is a need in the art to provide an economical andefficient process for fluxless reflow of solder bumped wafer underrelatively low temperatures to reduce thermal energy. There is a furtherneed in the art to provide a process and apparatus for fluxless reflowunder near ambient or atmospheric pressure conditions to avoid theexpense of purchasing and maintaining vacuum equipment. Additionally,there is a need in the art to provide a fluxless reflow process using anon-flammable gas environment.

BRIEF SUMMARY OF THE INVENTION

The present invention satisfies some, if not all, of the needs of theart by providing an apparatus and method for removing metal oxides fromthe surface of a substrate without the use of a flux. Specifically, inone aspect of the present invention, there is provided a method ofremoving a metal oxide from a treating surface of a substrate, themethod comprising: providing a substrate which is proximal to a baseelectrode having a grounded electrical potential, the substratecomprising a treating surface comprising the metal oxide; providing anenergizing electrode that is proximal to the base electrode and thesubstrate, wherein at least a portion of the treating surface is exposedto the energizing electrode and wherein the base electrode and theenergizing electrode and the substrate reside within a target area,wherein the energizing electrode is defined by an insulated platecomprising an array of protruding conductive tips, wherein theconductive tips are electrically connected by a conductive wire, whereinthe array of protruding tips is separated into a first electricallyconnected group and a second electrically connected group wherein one ofthe first or second electrically connected group is connected to a DCvoltage source that is positively biased and the other of the first orsecond electrically connected group is connected to a DC voltage sourcethat is negatively biased, and wherein the DC voltage source that ispositively biased and the DC voltage source that is negatively biasedare electrically connected to a functional controller that is capable ofalternating a supply of energy between the DC voltage source that isnegatively biased and the DC voltage source that is positively biased;passing a gas mixture comprising a reducing gas through the target area;energizing the rows of conductive tips by activating the DC voltagesource that is negatively biased to generate electrons within the targetarea, wherein at least a portion of the electrons attach to at least aportion of the reducing gas thereby forming a negatively chargedreducing gas; contacting the treating surface with the negativelycharged reducing gas to reduce the metal oxides on the treating surfaceof the substrate; and energizing the rows of conductive tips byactivating the DC voltage source that is positively biased to retrieveexcess electrons from the treating surface, wherein the rows ofconductive tips electrically connected to the DC voltage source that isnegatively biased and the rows of conductive tips electrically connectedto the DC voltage source that is positively biased are not energized atthe same time.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIGS. 1 a and 1 b illustrate a voltage pulse on an emission and a baseelectrode, respectively;

FIGS. 2 a through 2 i is a schematic illustration of various electrodegeometries suitable for emission and/or retrieval of electrons;

FIG. 3 provides an example of one embodiment of the electrode suitablefor emission and/or retrieval of electrons employing a plurality oftips;

FIG. 4 provides an example of one embodiment of the electrode suitablefor emission and/or retrieval of electrons having a segmented assembly;

FIG. 5 provides an example of one embodiment of the present inventionillustrating removal of surface metal oxides in a wafer bumpingapplication;

FIG. 6 illustrates a particular embodiment of the present invention forremoving negatively charged ions on the substrate surface by changingthe electrode polarity during the reflow of wafer bumps;

FIGS. 7 a and 7 b illustrates the transportation of the charged speciesbetween two electrodes when the polarity of the two electrodes ischanged;

FIG. 8 provides an illustration of a particular embodiment of thepresent invention for removing electrons on the surface of a substrateby employing an additional electrode with a positive bias relative tothe base electrode;

FIGS. 9 a through 9 e provide various illustrations of particularembodiments of the present invention employing movement of at least oneelectrode with respect to the substrate;

FIGS. 10 a and 10 b provide an illustration of a unidirectional voltagepulse and a bidirectional voltage pulse, respectively;

FIG. 11 is an illustration of an electrode assembly embodiment of thepresent invention;

FIGS. 12( a) and 12(b) are illustrations of certain features of theembodiment of FIG. 11;

FIG. 13 is an illustration of a conductive tip employed in theembodiment of FIG. 11; and

FIG. 14 is a diagram of the electrical arrangement of the conductivetips employed in the embodiment of FIG. 11.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method for the removal of metaloxides from a substrate surface by exposure to negatively charged ions.The negatively charged ions react and reduce the surface metal oxides.The present invention can be employed by modifying traditional reflowand soldering equipments such as, for example, the reflow machine usedfor reflow of solder bumped wafer. The present invention can also beapplied to other processes wherein the removal of the surface metaloxides from a substrate is desired such as, but not limited to, metalplating (i.e., the solder plating of portions of printed circuit boardsor metal surfaces to make them more amenable to subsequent soldering),reflow and wave soldering for outer lead bonding, surface cleaning,brazing, welding, and removing surface oxides of metals, such as copperoxide, formed during silicon wafer processing. The removal of metaloxides using the method and apparatus of the present invention isequally applicable to the aforementioned processes or any other processdesirous of removing oxides without the need for organic fluxes.

The term “substrate” as used herein generally relates to a material suchas silicon, silicon coated with silicon dioxide, aluminum-aluminumoxide, gallium arsenide, ceramic, quartz, copper, glass, epoxy, or anymaterial suitable for use within an electronic device. In certainembodiments, the substrate is an electrically insulated orsemi-conductive substrate having solder disposed thereupon. Exemplarysolder compositions include, but are not limited to, a fluxlesstin-silver, a fluxless tin-silver-copper, a fluxless tin-lead, or afluxless tin-copper. However, the method of the present invention issuitable for a variety of different substrates and solder compositions.In a certain preferred embodiment, the substrate is a silicon waferhaving a plurality of solder bumps disposed thereupon.

While not wishing to be bound by theory, it is believed that when anenergy source such as a direct current voltage source is applied betweenat least two electrodes, electrons are generated from one of the twoelectrodes having a negative electrical bias relative to the otherelectrode (referred to herein as an “emission electrode”) and/or the gasphase between the two electrodes. The generated electrons drift towardthe other electrode, which is grounded or has a positive electrical bias(referred to herein as a “base electrode”), along the electric field.The substrate having a plurality of solder bumps upon its surface isplaced within the area defined by the base and the emission electrodes(referred to herein as the “target area”) with the solder-bumped surfaceor treating area exposed to the emission electrode. In certainembodiments, the substrate may be connected to the base electrodeforming a target assembly. A gas mixture comprising a reducing gas andoptionally a carrier gas is passed through the electric field generatedby the electrodes. During the electron drift, part of the reducing gasforms negative ions by electron attachment which then drift to thetarget assembly, i.e., the base electrode and substrate surface. On thesubstrate surface, negatively charged ions can thus reduce the existingmetal oxides without the need for traditional fluxes. Further, theadsorption of the active species on the surface to be treated can bepromoted due to the drifting of the negatively charged ions along theelectric field.

In embodiments wherein the reducing gas comprises hydrogen, it isbelieved that the method of the present invention occurs as follows:

Dissociative Attachment of Molecular H₂: H₂+e′

H⁻+H   (I)

Electron Attachment on Hydrogen Atom: e′+H

H⁻  (II)

The combination of (I) and (II): 2e′+H₂

2H⁻  (III)

Oxide Reduction: 2H⁻+MO

M+H₂O+2e′ (M=solder/base metal)   (IV)

In these embodiments, the activation energy of metal oxide reductionusing the electron attachment method of the present invention is lowerthan methods that use molecular hydrogen because the formation of atomichydrogen ions with electron attachment eliminates the energy associatedwith bond breaking of molecular hydrogen.

In certain embodiments, energy is supplied to at least one of theelectrodes, preferably the emission electrode, sufficient to cause theemission electrode to generate electrons. The energy source ispreferably an electric energy or voltage source, such as an AC or DCsource. Other energy sources, such as an electromagnetic energy source,a thermal energy source, or a photo energy source may also be usedalone, or in combinations with any of the aforementioned energy sources.In certain embodiments of the present invention, the emission electrodeis connected to a first voltage level and the base electrode isconnected to a second voltage level. The difference in the voltagelevels creates a bias in electrical potential. One of the first or thesecond voltage levels may be zero indicating that either of the twoelectrodes can be grounded.

To produce negatively charged ions by electron attachment, a largequantity of electrons needs to be generated. In this connection, theelectrons can be generated by a variety of ways such as, but not limitedto, cathode emission, gas discharge, or combinations thereof. Amongthese electron generation methods, the selection of the method dependsmainly on the efficiency and the energy level of the electronsgenerated. For embodiments wherein the reducing gas is hydrogen,electrons having an energy level approaching 4 eV is preferred. In theseembodiments, such low energy level electrons are preferably generated bycathode emission rather than gas discharge. The generated electrons maythen drift from the emission electrode toward the base electrode whichcreates a space charge. The space charge provides the electron sourcefor generating the negatively charged hydrogen ions by electronattachment when hydrogen passes through the at least two electrodes orwithin the target area.

For embodiments involving electron generation through cathode emission,these embodiments may include: field emission (referred to herein ascold emission), thermal emission (referred to herein as hot emission),thermal-field emission, photoemission, and electron or ion beamemission.

Field emission involves applying an electric field with a negative biason the emission electrode relative to the base electrode that issufficiently high in intensity to overcome an energy barrier forelectrons to be generated from the surface of the emission electrode. Incertain preferred embodiments, a DC voltage is applied between the twoelectrodes that ranges from 0.1 to 50 kV, preferably ranging from 2 to30 kV. In these embodiments, the distance between the electrodes mayrange from 0.1 to 30 cm, preferably from 0.5 to 5 cm.

Thermal emission, on the other hand, involves using a high temperatureto energize electrons in the emission electrode and separate theelectrons from the metallic bond in the material of the emissionelectrode. In certain preferred embodiments, the temperature of theemission electrode may range from 800 to 3500° C., preferably from 800to 1500° C. The emission electrode may be brought to and/or maintainedat a high temperature by a variety of methods such as, but not limitedto, directly heating by passing AC or DC through the electrode; indirectheating such as contacting the cathode surface with an electricallyinsulated hot surface heated by a heating element, IR radiation, orcombinations thereof.

Thermal-field emission is a hybrid of field emission and thermalemission methods for electron generation in which both an electric fieldand a high temperature are applied. Therefore, thermal-field emissionmay require a lesser electric field and a lower electrode temperaturefor generating the same quantity of electrons as compared with purefield emission and pure thermal emission. Thermal-field emission canminimize difficulties encountered with pure field emission, such as thetendency of degradation in electron emission by contamination on theemission surface, and a high restriction on the planarity and uniformityof the emission surface. Further, thermal-field emission may also avoidproblems related to thermal emission such as a high potential ofchemical reaction between the emission electrode and the gas phase. Inembodiments wherein the thermal-field emission is used for electrongeneration, the temperature of the emission electrode can range fromambient to 3500° C., or more preferably from 150 to 1500° C. In theseembodiments, the electric voltage can range from 0.01 to 30 KV, or morepreferably from 0.1 to 10 KV.

In certain preferred embodiments, the thermal emission or thermal-fieldemission method is used for electron generation. In these embodiments, ahigh temperature emission electrode used in either of these methods mayalso act as a heat source for the gas mixture that is passed through theelectric field generated by the two electrodes, so that the thermalenergy required for heating the gas for a subsequent reflow process stepcan be reduced.

In certain embodiments of the present invention, the electron generationis accomplished via a combination of cathode emission and coronadischarge methods. In these embodiments, an energy source such as a DCvoltage is applied between the two electrodes and electrons may begenerated from both the emission electrode (cold or hot) and the gasphase (corona discharge) near the emission electrode. The coronadischarge is preferably minimized in order to increase the efficiency offorming negatively charged ions by electron attachment and increase thelifetime of the emission electrode.

In embodiments wherein the cathode emission mechanism is used forelectron emission, the voltage applied across the two electrodes may beconstant or pulsed. The frequency of the voltage pulse ranges from 0 to100 kHz. FIGS. 1 a and 1 b provide an illustration of a voltage pulse onan emission electrode and a base electrode, respectively. In theseembodiments, it is believed that a pulsed voltage, such as that shown inFIGS. 1 a and 1 b, is preferable to constant voltage to improve theamount of electron generation and to reduce the tendency of gas phasedischarge.

For embodiments involving electron generation through gas discharge,these embodiments may include thermal discharge, photo-discharge, andvarious avalanche discharge, including glow discharge, arc discharge,spark discharge, and corona discharge. In these embodiments, electronsare generated by gas phase ionization. The gas phase is a gas mixturecomprising the reducing gas and an inert gas. In certain embodiments ofgas phase ionization, a voltage source is applied between two electrodesand electrons may be generated from the inert gas within the gas mixturebetween the two electrodes that then drift toward the positively biasedelectrode such as the base electrode. During this electron drift, someof these electrons may attach on the reducing gas molecules and formnegatively charged ions by electron attachment. In addition, somepositive ions are also created in the inert gas that then drift towardthe negatively biased electrode such as the emission electrode and areneutralized at the electrode surface.

As mentioned previously, electrons can be generated from an emissionelectrode when it has a negative bias relative to a base electrode.Referring to FIGS. 2 a through 2 i, the emission electrode may have avariety of geometries, such as, for example, a thin wire 2 a, a rod witha sharpened tip 2 b, a rod with several sharpened tips or comb 2 c, ascreen or wire mesh 2 d, a loose coil 2 e, an array of combs 2 f, abundle of thin wires or filament 2 g, a rod with sharp tips protrudingfrom its surface 2 h, or a plate with a knurled surface 2 i. Additionalgeometries may include combinations of the above geometries such asplates or rods with surface protrusions, rods wrapped with wire windingsor filament, coils of thin wires, etc. A plurality of electrodes may beemployed that may be arranged in a parallel series or in an intersectinggrid. Still further geometries may include a “wagon wheel” geometrywherein a plurality of electrodes is arranged in a radial fashion suchas in “spokes” of a wheel. In certain embodiments, such as embodimentswherein field emission is involved, the cathode is preferably made ofgeometries having a large surface curvature, such as a plurality ofsharp tips to maximize the electric field near the electrode surfacesuch as the geometry depicted in FIG. 3. As FIG. 3 illustrates,electrode 1 has a series of thin wires 2 that reside within grooves onthe electrode surface along with a plurality of tips 3 emanating fromits surface.

The electrode material that acts as an emission electrode is preferablycomprised of a conductive material with relatively low electron-emissionenergy or work function. The material preferably also has a high meltingpoint and relatively high stability under processing conditions.Examples of suitable materials include metals, alloys, semiconductors,and oxides coated or deposited onto conductive substrates. Furtherexamples include, but are not limited to, tungsten, graphite, hightemperature alloys such as nickel chromium alloys, and metal oxides suchas BaO and Al₂O₃that are deposited onto a conductive substrate.

The electrode that acts as a base electrode is comprised of a conductivematerial such as a metal or any of the other materials describedtherein. The base electrode can have a variety of different geometriesdepending upon the application.

In certain embodiments of the present invention involving thermal-fieldemission, the emission electrode may comprise a segmented assembly suchas the electrode depicted in FIG. 4. In this regard, the core 10 of theemission electrode may be made of a metal with a high electricalresistance. A plurality of tips 11 emanate from core 10. Tips 11 may bemade of a conductive material with relatively low electron emissionenergy or work function such as any of the materials disclosed herein.The core may be heated by directly passing an AC or a DC current (notshown) through core 10. The thermal conduction will transfer the heatfrom the core to tips 11. The hot core 10 and the plurality of tips 11are enclosed within an enclosure 12 which is then inserted into asupport frame thereby forming a segmented assembly as shown. Tips 11 areexposed outside the enclosure 12. Enclosure 12 is composed of aninsulating material. The segmented assembly allows for the thermalexpansion of the core during operation. In this arrangement, electronscan be generated from hot tips 11 by applying a negative voltage bias onthe emission electrode relative to the base electrode.

In another preferred embodiment of the present invention involvingthermal-field emission, indirect heating can raise the temperature ofthe emission electrode. This may be accomplished by using a heatingcartridge as the core of the emission electrode. The surface of theheating cartridge may be comprised of an electrically conductivematerial such as a metal that is electrically insulated from the heatingelement inside the cartridge. To promote electron emission, a pluralityof distributed emission tips can be mounted on the surface of theheating cartridge. The cartridge can be heated by passing an AC or DCcurrent through the heating element inside the cartridge. Electrons canbe generated from the distributed tips of the cartridge by applying anegative voltage bias on the surface of the cartridge relative to asecond electrode. For creating the voltage bias in this arrangement, thesecond electrode can be grounded so that the cartridge may be negativelybiased or, alternatively, the cartridge can be grounded so that thesecond electrode may be positively biased. In some embodiments, thelatter case may be preferable for eliminating a potential interferencebetween two electric circuits, e.g., one circuit may be the AC or DCcurrent along the heating element, and the another circuit may be thehigh voltage bias between the surface of the cartridge and the secondelectrode. In these embodiments, the hot cartridge electrode may alsoact as a heat source for the gas mixture to achieve the requiredtemperatures for the reflow process step.

As mentioned previously, a gas mixture comprising a reducing gas ispassed through the electric field generated by the at least twoelectrodes. The reducing gas contained within the gas mixture may fallwithin one or more of the following categories: 1) an intrinsicallyreductant gas, 2) a gas capable of generating active species which formgaseous oxides upon reaction of the active species with the metal oxide,or 3) a gas capable of generating active species which form liquid oraqueous oxides upon reaction of the active species with the metal oxide.

The first category of gases, or an intrinsically reductant gas, includesany gas that thermodynamically acts as a reductant to the oxides to beremoved. Examples of intrinsically reductant gases include H₂, CO, SiH₄,Si₂H₆, formic acid, alcohols such as, for example, methanol, ethanol,etc., and some acidic vapors having the following formula (III):

In formula (III), substituent R may be an alkyl group, substituted alkylgroup, an aryl, or substituted aryl group. The term “alkyl” as usedherein includes straight chain, branched, or cyclic alkyl groups,preferably containing from 1 to 20 carbon atoms, or more preferably from1 to 10 carbon atoms. This applies also to alkyl moieties contained inother groups such as haloalkyl, alkaryl, or aralkyl. The term“substituted alkyl” applies to alkyl moieties that have substituentsthat include heteroatoms such as O, N, S, or halogen atoms; OCH₃; OR(R=alkyl C₁₋₁₀ or aryl C₆₋₁₀); alkyl C₁₋₁₀ or aryl C₆₋₁₀; NO₂; SO₃R(R=alkyl C₁₋₁₀ or aryl C₆₋₁₀); or NR₂ (R═H, alkyl C₁₋₁₀ or aryl C₆₋₁₀).The term “halogen” as used herein includes fluorine, chlorine, bromine,and iodine. The term “aryl” as used herein includes six to twelve membercarbon rings having aromatic character. The term “substituted aryl” asused herein includes aryl rings having substitutents that includeheteroatoms such as O, N, S, or halogen atoms; OCH₃; OR (R=alkyl C₁₋₁₀or aryl C₆₋₁₀); alkyl C₁₋₁₀ or aryl C₆₋₁₀; NO₂; SO₃R (R=alkyl C₁₋₁₀ oraryl C₆₋₁₀); or NR₂ (R=H, alkyl C₁₋₁₀ or aryl C₆₋₁₀). In certainpreferred embodiments, the gas mixture contains hydrogen.

The second category of reducing gas includes any gas that is not anintrinsically reductive but can generate active species, such as, forexample, H, C, S, H′, C′, and S′, by dissociative attachment of electronon the gas molecules and form gaseous oxides by reaction of the activespecies with the metal oxides to be removed. Examples of this type ofgas include: NH₃, H₂S, C₁ to C₁₀ hydrocarbons such as but not limited toCH₄, C₂H₄, acidic vapors having the formula (III), and organic vaporshaving the following formula (IV):

In formulas (III) and (IV), substituent R may be an alkyl group,substituted alkyl group, an aryl, or substituted aryl group.

The third category of reducing gas includes any gas that is not anintrinsically reductive but can form active species, such as, forexample, F, Cl, F′, and Cl′, by dissociative attachment of electron onthe gas molecules and form liquid or aqueous oxides by reaction of theactive species with the metal oxides. Examples of this type of gasinclude fluorine and chlorine containing gases, such as CF₄, SF₆,CF₂Cl₂, HCl, BF₃, WF₆, UF₆, SiF₃, NF₃, CClF₃, and HF.

Besides including one or more of the above categories of reducing gases,the gas mixture may further contain one or more carrier gases. Thecarrier gas may be used, for example, to dilute the reducing gas orprovide collision stabilization. The carrier gas used in the gas mixturemay be any gas with an electron affinity less than that of the reducinggas or gases within the gas mixture. In certain preferred embodiments,the carrier gas is an inert gas. Examples of suitable inert gasesinclude, but are not limited to, N₂, Ar, He, Ne, Kr, Xe, and Rn.

In certain preferred embodiments, the gas mixture comprises hydrogen asthe reducing gas and nitrogen as the carrier gas due to its relativelylower cost and the environmental friendliness of the exhaust gasrelease. In these embodiments, the gas mixture comprises from 0.1 to100% by volume, preferably 1 to 50% by volume, or more preferably from0.1 to 4% by volume of hydrogen. Amounts of hydrogen lower than 4% arepreferred, which makes gas mixture non-flammable.

In certain embodiments, the gas mixture is passed through the fieldgenerated by the at least two electrodes at a temperature ranging fromambient to 450° C., more preferably ranging from 100 to 350° C. Thepressure of the gas mixture is preferably ambient atmospheric pressure,i.e., the existing pressure of the area of the process. No specialpressure, such as vacuum, may be required. In embodiments where the gasmixture is pressurized, the pressure may range from 10 to 20 psia,preferably from 14 to 16 psia.

The substrate surface in which the oxides are to be removed ispreferably located between the emission electrode and the base electrodewith the surface facing the emission electrode. In certain preferredembodiments of the present invention, the substrate may be connected tothe base electrode to provide a target assembly and facing the emissionelectrode. In these embodiments, the distance between the emissionelectrode and the top surface of the wafer/or target assembly may rangefrom 0.1 to 30 cm, preferably from 0.5 to 5 cm.

FIG. 5 provides an illustration of the process used for a wafer bumpingapplication wherein the substrate is silicon wafer 20. Referring to FIG.5, a second electrode 24 is located above a wafer 20, and the wafer 20comprising a plurality of solder bumps (not shown) is placed upon afirst electrode 22 to form a target assembly. At least a portion of thesurface of wafer 20 containing the plurality of solder bumps is exposedto second electrode 24. While the wafer 20 is shown as being placed uponfirst electrode 22, it is envisioned that the wafer 20 can be placedanywhere between electrodes 22 and 24. A pulsed voltage 25 is appliedacross the first electrode 22 and the second electrode 24 to generate anelectric field. A gas mixture 26 containing hydrogen and nitrogen ispassed through the electric field. Low energy electrons 28 are generatedwithin the electric field which drift towards the first electrode 22 andwafer 20 disposed thereupon. In addition, a portion of the hydrogenwithin gas mixture 26 forms hydrogen ions 30 by electron attachmentwhich also drift towards the first electrode 22 and wafer 20 disposedthereupon. The drift of negatively charged hydrogen ions 30 andelectrons 28 towards the electrode 22 with wafer 20 disposed thereuponpromotes adsorption of ions 30 onto the surface of wafer 20 and fostersde-oxidation of the wafer surface (referred to herein as surfacede-oxidation).

Depending upon the conductivity of the substrate, some of the electronsthat are generated as a reaction byproduct from surface de-oxidation canaccumulate on the substrate surface. In addition, a portion of the freeelectrons can directly adsorb on the substrate due to the drifting alongthe electric field. This electron build-up on the substrate surface mayprevent additional adsorption of the negatively charged ions as well asadversely affect the equilibrium of the surface de-oxidation. To makethe surface de-oxidation process more efficient, the electrons on thesubstrate surface need to be periodically removed.

One method to remove the electrons on the substrate surface may be tochange the polarity of both of the electrodes relative to each other.During the polarity change, the voltage level on each electrode may notnecessarily be the same. In one embodiment, the polarity change can beachieved by applying a bi-directional voltage pulse between at least twoelectrodes such as that shown in FIG. 10 b. FIG. 6 provides an exampleof a polarity change wherein an electrode may generate electrons in onephase of the voltage pulse (i.e., a negative bias) and retrieveelectrons in another phase of the voltage pulse (i.e., a positive bias).In FIG. 6, electrode 110 is used as both the electron emission andelectron retrieving electrode and electrode 120 is used a baseelectrode. This arrangement allows the efficiency of surfacede-oxidation to be maximized. Electrode 110 containing a plurality ofsharp tips 101 is located above wafer 103. Electrode 110 is heated byconnecting to an AC power source 104. Another electrode 120 is locatedunderneath wafer 103. The change in polarity of electrodes 110 and 120can be obtained, for example, by a bidirectional pulsed DC power 105. Anexample of bi-directional voltage pulse is illustrated in FIG. 10 b.When electrode 110 is negatively biased, at least a portion of theelectrons generated from tips 101 attach to at least a portion of thereducing gas and newly created reducing gas ions drift towards wafer103. When the polarity is reversed, electrons are released from thesurface of wafer 103 and retrieved back at tips 101. FIGS. 7 a and 7 billustrate the transportation of the charged species during each cycleof the voltage pulse. The frequency of changing the polarity of the twoelectrodes can range from 0 to 100 kHz.

In an alternative embodiment, excess electrons on the substrate surfacemay be removed by employing one or more additional electrodes. FIG. 8provides such an example wherein a wafer is the substrate. Referring toFIG. 8, wafer 200 is disposed upon a grounded base electrode 201. Twoelectrodes, electrode 202 having a negative voltage bias relative tobase electrode 201 and electrode 203 having a positive voltage biasrelative to base electrode 201 are installed above the wafer surface200. In this arrangement, electrons are continually generated fromelectrode 202 and retrieved at electrode 203. In one particularembodiment, the polarity of electrode 202 and electrode 203 may beperiodically changed from a positive to a negative voltage bias, andvice versa, relative to the base electrode 201.

In yet another embodiment, electrons or residual surface charge may beremoved from the substrate surface by employing a neutralizer aftersurface de-oxidation. If left untreated, residual charge contaminationcan cause electrostatic discharge damage to sensitive electroniccomponents. In these embodiments, flowing a high purity gas such as N₂through a commercially available neutralizer device and then across thesubstrate surface can neutralize the residual charge on the wafersurface. The positive ions present in the gas will neutralize anyresidual electrons and provide an electrically neutral surface. Suitablecharge neutralizer devices may, for example, consist of Kr-85radioactive sources that produce equal densities of positive andnegative ions in the gas. While positive and negative ions are producedin the gas as it flows through the wafer, the net charge of the gasstream is zero.

FIGS. 11 to 14 illustrate a preferred electrode assembly apparatusemployed for removing a metal oxide from a treating surface of asubstrate according to the present invention. Referring to FIG. 11,electrode assembly comprises energizing electrode 501 having an array ofprotruding conductive tips 506 that are electrically connected by aconductive wire (not shown), and base electrode 502, which has agrounded electrical potential. As used herein the term “energizingelectrode” refers to an electrode that is an electrode that has anelectrical potential—either positive or negative—relative to ground(zero). The energizing electrode 501 is proximal to the base electrode502. Preferably, the base electrode 502 and the energizing electrode 501are spaced apart from one another by a distance of from about 0.5 cm toabout 5.0 cm. In preferred embodiments, the base electrode 502 and theenergizing electrode 501 are spaced apart from one another by a distanceof 1.0 cm.

A substrate 503 is proximal to the base electrode 502. Substrate 503comprises an insulating portion 504 and a treating surface 505comprising a metal oxide. Preferably, at least a portion of the treatingsurface 505 is exposed to the energizing electrode 501 and the baseelectrode 502 and the energizing electrode 501 and the substrate 503 allreside within a target area.

Referring now to FIGS. 11 and 12( a), energizing electrode 501 isdefined by an insulated plate having an array of protruding conductivetips 506 that are electrically connected by a conductive wire.Preferably, the insulated plate comprises a material selected frominsulating materials such as, for example, quartz, a ceramic material, apolymer, and mixtures thereof. In preferred embodiments, the insulatingplate comprises quartz.

In preferred embodiments of the present invention the protrudingconductive tips are removably connected to the base plate. For example,referring to FIG. 12( a), the underside of the insulated plate has holes507 that resemble “key holes.” Each of the holes 507 is connected by aconductive wire 508, which electrically contacts the conductive tips 506once the conductive tips 506 are inserted into the holes 507. FIG. 12(b) shows a close-up of the arrangement of three of the holes 507.Preferably, the holes 507 (and, thus, the conductive tips 506) arespaced apart from each other by a distance of from about 2 mm to about10 mm, more preferably from about 5 mm to about 8 mm, and mostpreferable, by a distance of about 5 mm. The holes 507 can be formed byany means known to those skilled in the art such as, for example, bylaser drilling.

FIG. 13 shows a single conductive tip 506 whose upper end 508 fits intoone of the holes 507, contacts conductive wire 508, and locks into placeby, for example, a twisting motion. Each of the conductive tips can beunlocked and removed by, for example, a twisting motion in the oppositedirection than that used to lock the conductive tip 506 into place.Preferably, conductive tips 506 are formed from metals such as, forexample, brass, stainless steel, nickel-chromium alloy, and other metalswith relatively low electron-emission energy.

Referring now to FIG. 14, the array of protruding conductive tips 506 isseparated into a first electrically connected group 510 and a secondelectrically connected group 512 wherein one of the first or secondelectrically connected group is connected to a DC voltage source that ispositively biased 516 and the other of the first or second electricallyconnected group is connected to a DC voltage source that is negativelybiased 518. The DC voltage source that is positively biased and the DCvoltage source that is negatively biased are electrically connected to afunctional controller 514 that is capable of alternating a supply ofenergy between the DC voltage source that is negatively biased and theDC voltage source that is positively biased. Between the functionalcontroller 514 and each of the positive and negative DC voltage sourcesis a positive pulse generator 520 and a negative pulse generator 522 incommunication with the positive and negative DC voltage sources,respectively. The pulse generators 520 and 522 are, in effect, switches.When each pulse generator receives a pulsing signal (e.g., a pulsed lowvoltage signal of 0 to 5v) from the functional controller 514, acorresponding high voltage pulse (e.g., between 0v and the set point ofthe DC voltage) will be created, which powers the corresponding highvoltage source. The functional controller 514 has a dual outputs. Whenone output is connected to a corresponding pulse generator, the otheroutput is disconnected to the other pulse generator, thus making thealternating function. Typically, there are two adjustable variables ofthe pulsing signal of the functional controller: pulsing frequency andthe on/off ratio of the voltage.

In operation, when a gas mixture comprising a reducing gas is passedthrough the target area, the functional controller 514, in cooperationwith the negative pulse generator 522, energizes the rows of conductivetips by activating the DC voltage source that is negatively biased togenerate electrons within the target area, wherein at least a portion ofthe electrons attach to at least a portion of the reducing gas therebyforming a negatively charged reducing gas that, in turn, contacts thetreating surface to reduce metal oxides on the treating surface of thesubstrate. The functional controller 514 then deactivates the DC voltagesource that is negatively biased and, in cooperation with the positivepulse generator 520, activates the DC voltage source that is positivelybiased to energize the other set of rows of conductive tips to retrieveexcess electrons from the treating surface. Preferably, the rows ofconductive tips electrically connected to the DC voltage source that isnegatively biased and the rows of conductive tips electrically connectedto the DC voltage source that is positively biased are not energized atthe same time. In operation, functional controller 514 is capable ofalternating between the negative and positive biased DC voltage at ahigh-frequency pulse. In preferred embodiments, the voltage pulsingfrequency is preferably from about 0 to about 50 kHz and, morepreferably, from about 10 kHz to about 30 kHz. The amplitude of thevoltage pulse is preferably from about 1 kV to about 3 kV.

There are several benefits of the embodiment of the present inventionillustrated by FIGS. 11 to 14. First, such embodiment allows for the lowtemperature reduction of metal oxides such as, for example, copperoxide, on an insulated substrate such as, for example, a ceramicsubstrate. Examples of insulated substrates include, for example, rigidepoxy glass laminate substrates used for printed circuit boards,flexible flex polymeric films (e.g., polyimide) substrates used forflexible circuits, insulated substrates used in integrated circuitinterconnection schemes, high density interconnects such as polymericbased BGA's (Ball Grid Array), high density interconnects, and insulatedsubstrates used in 3-D or stacked integrated circuit or 3-D or stackedpackaging technologies. Next, the embodiment illustrated in FIGS. 11 to14 provides a structure and electrical arrangement that is able tointensify the electrical field on each conductive tip, thereby largelyreducing the threshold voltage for a uniform electron emission among thetips without arcing between adjacent tips of opposite voltagepotentials.

In certain embodiments, the substrate or target assembly may be movedwith respect to the electrode that acts as an emission electrode. Inthis regard, the emission electrode may be in a fixed position and thesubstrate may be moved, the emission electrode may be moved and thesubstrate may be in a fixed position, or both the emission electrode andthe substrate are moved. The movement may be vertical, horizontal,rotational, or along an arc. In these embodiments, surface de-oxidationmay then occur within a localized area of the substrate surface.

For the following FIGS. 9 a through 9 e, the substrate is a siliconwafer that is disposed upon a base electrode, which is grounded. Atleast portion of the wafer surface comprising a plurality of solderbumps (not shown) is exposed to a second electrode that acts as both anemitting and a retrieving electrode (i.e., changing polarity, forexample, from a negative to a positive bias in electrical potential).FIG. 9 a shows a silicon wafer 300 moved rotationally under a heatedlinear electrode 302 to which is applied a bi-directionally pulsedvoltage relative to the base electrode 304 that may cycled in afrequency range from 0 to 100 KHz thereby generating an ion area shownas 306. A motor located outside the processing chamber (not shown)rotates the wafer. Such rotation is frequently accomplished insemiconductor processing without significant contamination of the wafersurface. Contamination may be avoided by employing high cleanliness,rotational feed-throughs and control of flow patterns. FIG. 9 b shows asilicon wafer 310 moved linearly under a heated linear electrode 312 towhich is applied a bi-directionally pulsed voltage relative to the baseelectrode 314 thereby generating an ion area shown as 316. Thisarrangement may be suitable for applications wherein a conveyor belt isused to move wafers through tubular bumping furnaces such as, forexample, printed circuit boards through reflow furnaces. FIG. 9 c showsa silicon wafer 320 moved rotationally under a pair of heated linearemission electrodes 324 and 326: the base electrode 322 has a steadypositive bias and the emission electrodes 324 and 326 has a steadynegative bias relative to the base electrode 322 so that electrons canbe generated to and be retrieved back from the wafer surface therebygenerating an ion area shown as 328. A motor located outside theprocessing chamber (not shown) rotates the wafer. FIG. 9 d shows asilicon wafer 330 moved linearly under a heated pair of linearelectrodes 334 and 336 that are held at steady and opposite polaritiesrelative to the base electrode 332 to conduct electron emission andretrieving separately_thereby generating an ion area shown as 338.Lastly, FIG. 9 e employs a relatively smaller electrode 344 located onthe end of a pivoting arm 346. The polarity of the electrode 344 isfrequently altered relative to the base electrode 342 thereby generatingan ion area shown as 348. The arm is swung over a rotating wafer 340, infor example an arc-like motion, to affect complete and uniform treatmentof the entire wafer surface.

The method of the present invention can be used in several areas of themicroelectronics manufacturing in addition to the reflow of a solderbumped wafer such as, for example, surface cleaning, metal plating,brazing, welding, and reflow and wave soldering for outer lead bonding.An example of reflow and wave soldering apparatuses that are suitablefor the method of present invention is in FIGS. 1-3 provided in pendingU.S. application Ser. No. 09/949,580 which assigned to the assignee ofthe present invention and is incorporated herein by reference it itsentirety. In one particular embodiment, the method of the presentinvention can be used to reduce surface oxides of metals, such as copperoxide, formed during silicon wafer processing or thin film de-oxidation.Such oxides may form as a result of the various wet processing steps,such as chemical mechanical planarization, that are used to formmicro-electronic devices on the wafers. These surface oxides reducedevice yield and device reliability. The present invention allowssurface oxides to be removed in a fully dry, environmentally friendlymanner that does not require the use of aqueous reducing agents.Further, since the present invention is performed at relatively lowtemperatures, it does not significantly affect the thermal budget of thedevice during processing. Higher temperatures, by contrast, tend toreduce device yield and reliability by causing diffusion of dopants andoxides thereby reducing device performance. Since the method of thepresent invention can be performed on a single wafer, the method can beintegrated with other single wafer processes, thereby providing bettercompatibility with other fabrication steps.

The method and apparatus of the present invention are particularlysuitable for applications of wafer bumping and thin film de-oxidation.The conveniences of using the present invention for wafer bumping andthin film de-oxidation are numerous. First, compared to typical reflowsoldering processes for outer lead bonding, wafer bumping and thin filmde-oxidation are both single-face treatment. In this connection, thespace above the surface to be deoxidized can be as small as 1 cm,thereby resulting in an efficient process for both ion generation andtransportation. Second, the processing temperatures for reflow in waferbumping are significantly higher than that of typical reflow solderingprocesses. The higher temperatures promote the formation of thenegatively charged ions by electron attachment. Third, in wafer bumpingand thin film de-oxidation processes, the solder bumps and thin filmsare completely exposed thereby minimizing any “shadow” effect duringsurface de-oxidation. Further, compared to other soldering processeswhereby the solder has to wet and spread over the component surface, thedeposited solder bumps on the wafer need only exhibit solder ballformation upon first reflow.

The invention will be illustrated in more detail with reference to thefollowing examples, but it should be understood that the presentinvention is not deemed to be limited thereto.

EXAMPLES Example 1

A first experiment was conducted by using a lab-scale furnace. Thesample used was a fluxless tin-lead solder preform (melting point 183°C.) on a grounded copper plate (anode), which was loaded inside afurnace and heated up to 250° C. under a gas flow of 5% H₂ in N₂. Whenthe sample temperature was at equilibrium, a DC voltage was appliedbetween the negative electrode (cathode) and the grounded sample (anode)and gradually increased to about −2 kV with a current of 0.3 mA. Thedistance between the two electrodes was about 1 cm. The pressure wasambient, atmospheric pressure. It was found that the solder was indeedvery well wetted on the copper surface. Without applying an electricvoltage, a good wetting of a fluxless solder on a copper surface cannever be achieved at such low temperature, even in pure H₂, because theeffective temperature for pure H₂ to remove tin-oxides on a tin-basedsolder is above 350° C. Therefore, this result confirms that theelectron-attachment method is effective in promoting H₂ fluxlesssoldering.

Example 2

Several cathode materials were investigated for electron-attachmentassisted hydrogen fluxless soldering by using the field emissionmechanism using the same set-up as Example 1. The results of theinvestigation is provided in Table I.

As Table I illustrates, the best result was obtained by using a Ni/Crcathode, which provided the highest fluxing efficiency and thus resultedin the shortest wetting time. It is believed that the Ni/Cr cathodegenerates a relatively larger quantity of electrons and has suitableenergy level of the electrons compared to other cathode materials.

TABLE I Effect of Cathode Material on Wetting Time at 250° C. and 20% H₂Material of Cathode Rod with a Sharp Tip ( 1/16″ dia.) Time to CompleteWetting Brass 1 min 55 sec Copper 1 min 44 sec Nickel Chromium 39 secAluminum 1 min 28 sec Stainless Steel 1 min Tungsten 1 min 54 sec

Example 3

The present example was conducted to investigate the effectiveness ofthe thermal-field emission method for generating electrons. A 3 mmdiameter graphite rod, having a number of 1 mm long machined tipsprotruding from its surface, acted as the cathode and had a geometrysimilar to that depicted in FIG. 2 i. Each of the protruding machinedtips had a tip angle of 25 degrees. The graphite rod was heated up in agas mixture of 5% H₂ and 95% N₂ to about 400 to 500° C. by resistiveheating using an AC power source. A DC voltage source of 5 KV wasapplied between the graphite cathode and a copper plate that acted as ananode having a 1.5 cm gap therebetween. All the tips on the graphite rodwere illuminated thereby indicating that electrons could uniformly begenerated from the distributed tips on the graphite rod. Without heatingof the graphite rod, there would be either no electron emission from thecathode, or arcing between one of the tips and the anode plate. Thisdemonstrates that the combination of using a cathode having multipletips and elevated temperatures, i.e., a thermal-field emission method,is effective for obtaining uniform electron emission from an integratedemitting system.

Example 4

The present example was conducted using a 0.04″ diameter nickel-chromiumalloy heating wire clamped horizontally between two machined Al₂O₃refractory plates such as the electrode illustrated in FIG. 4. A seriesof five nickel-chromium alloy emitting wires, each with a sharp tip(12.5 degree) on one end of the wire, protruded perpendicularly from thenickel-chromium heating wire and were vertically positioned between tworefractory plates. The nickel-chromium heating wire and tips were heatedup in a gas mixture of 5% H₂ and 95% N₂ to about 870° C. using an ACpower source. A DC voltage of 2.6 KV was applied between the cathode anda copper plate that acted as the anode having a 6 mm gap between the twoelectrodes. All five tips were illuminated and the total emissioncurrent reached 2.4 mA. Without heating of the wire, there would beeither no electron emission from the cathode, or arcing between one ofthe tips and the anode plate. Like example 3, example 4 demonstratesthat thermal-assisted field emission provides uniform electron emission.Further, because of the higher temperature of the emission electrode, italso increases the quantity of the electron emission at a given electricpotential.

Example 5

The present example was conducted to demonstrate the effect of a voltagepulse between two electrodes on cathode emission. A single-tipnickel-chromium alloy wire was used as the emission electrode and agrounded copper plate acted as a base electrode. The copper plate waslocated 3 mm below the tip of the emission electrode. A tin/lead solderpreform was disposed upon the copper plate. The nickel-chromium wire,preform, and copper plate were maintained in a furnace at ambienttemperature in a gas mixture of 4% H₂ and the remainder N₂. A pulsedunidirectional voltage of various frequencies and amplitudes was appliedbetween the two electrodes. In this connection, the electric potentialof the emission electrode was varied from negative to zero relative tothe grounded base electrode thereby allowing electrons to be generatedfrom the tip electrode. The results are provided in Table II.

The results in Table II indicate that greater quantities of electronsare generated from the emission electrode when a voltage pulse of higherpulsing frequency and amplitude is applied.

TABLE II Uni-Directional Voltage Pulse Pulsing Frequency (Hz) 0 250 5001000 2500 Emission Current at 3.4 kV 0 0.3 0.4 0.5 0.6 Pulsing Amplitude(mA) Emission Current at 1.0 kV 0 0.1 0.1 0.2 0.2 Pulsing Amplitude (mA)

Example 6

The present example was conducted to demonstrate surface discharge byaltering the polarity of the two electrodes using the same set-up asexample 5.

A bidirectional voltage pulse with a total pulsing amplitude of 3.4 kV(e.g. from +1.7 kV to −1.7 kV) was applied between the two electrodes.During the bi-directional voltage pulse, the polarity of the twoelectrodes was changed. In other words, the tip of the emissionelectrode was varied from a positive to a negative electrical biasrelative to the grounded base electrode thereby allowing electrons to begenerated from and retrieved to the tip electrode.

Table III provides the leakage current from the base electrode for eachfrequency of the polarity change. As Table III illustrates, the higherthe frequency of the polarity change, the lower the charge build up willbe by observing the leakage current passing through the copper baseelectrode.

TABLE III Bi-Directional Voltage Pulse Pulsing Frequency (Hz) 250 5001000 2500 Leakage Current (mA) 0.00069 0.00054 0.00015 0.00015

Example 7

The present example was conducted to demonstrate remote surfacedischarge by employing an additional electrode. A 90 Pb/10Sn solderpreform having a melting point of 305° C. was set on a small piece of acopper substrate that was set on an electrically insulated wafer. Agrounded copper plate was placed underneath the wafer and acted as abase electrode. Two single-tip nickel-chromium wires, one with anegative voltage and one with a positive voltage, were installed 1 cmabove the base electrode with the solder preform. The distance betweenthe two single-tip electrodes was 1.5 cm. The arrangement was heatedwithin a gas mixture containing 4% H₂ in N₂ from room temperature to agiven reflow temperature above the melting point of the solder. When thereflow temperature reached equilibrium, electron attachment was startedby applying the positive and the negative voltages to the two single-tipelectrodes and the time required for the solder preform to form aspherical ball was recorded. The formation of a spherical solder ballindicated an oxide-free solder surface. As shown in Table IV, thesurface de-oxidation is quite efficient at a range of temperature from310 to 330° C., which is only 5 to 15° C. above the melting point of thesolder.

TABLE IV Isotherm Reflow Temperature 310 320 330 (° C.) Ball formationTime (seconds) 20, 18, 20, 24 17, 13, 16 14, 12 Average Ball FormationTime  20.5  15.3  13 (seconds)

Example 8

A quartz-plate based electrode according to the present invention wasinstalled inside a furnace. The space between metal pins on the quartzplate was 0.5 cm. The substrate was a copper plate on a ceramicsubstrate which, in turn, was put on a grounded electrode (i.e., a baseelectrode). The gap between the tips of metal pins on the quartz plateand the copper surface was 1 cm. The furnace was purged by 5% H₂balanced with N₂. A pulsed voltage ranging from 0 to −3.12 kV with apulsing frequency of 10 kHz was applied on the quartz electrode. Auniform electron emission from all the emission pins was observed. Theaverage emission current from each emission pin was around 0.3 mA.Significantly, no arcing was observed in this embodiment where the spacebetween the emission tips was as small as 0.5 cm. This resultdemonstrates that the threshold voltage for a uniform electron emissionfrom the quartz-plate based electrode is largely reduced to a level muchbelow that of gas ionization.

Example 9

A quartz-plate based electrode according to the present invention wasinstalled inside a furnace. The space between metal pins on the quartzplate was 1.0 cm. The treating surface was a copper plate that had beenpre-oxidized in air at 150° C. for two hours, which made the color ofthe copper plate darker compared with original copper plate. The oxidethickness was estimated to be ˜400 Å by Auger analysis. The pre-oxidizedcopper plate was put on a ceramic substrate and then located on agrounded electrode. The gap between the tips of metal pins on quartzplate and the surface of the pre-oxidized copper plate was 1 cm. Thefurnace was purged by 5% H₂ balanced with N₂ and heated to 200° C.Positive and negative voltages were alternatively applied on twodifferent groups of metal pins with amplitudes of +1.2 kV and −1.2 kV,respectively. The alternating frequency was 15 KHz. A uniform electronemission was observed. After a 15 minutes of such electron attachmenttreatment under the condition described above, the copper plate wastaken out of the furnace. It was observed that the color of the treatedcopper surface was changed to the original copper plate color. Withoutapplying the electron attachment process of the present invention, theoxide reductions on the surface of the pre-oxidized copper were found tobe inefficient in the same heating process.

While the invention has been described in detail and with reference tospecific examples thereof, it will be apparent to one skilled in the artthat various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

1. A method of removing a metal oxide from a treating surface of asubstrate, the method comprising: providing a substrate which isproximal to a base electrode having a grounded electrical potential, thesubstrate comprising a treating surface comprising the metal oxide;providing an energizing electrode that is proximal to the base electrodeand the substrate, wherein at least a portion of the treating surface isexposed to the energizing electrode and wherein the base electrode andthe energizing electrode and the substrate reside within a target area,wherein the energizing electrode is defined by an insulated platecomprising an array of protruding conductive tips, wherein theconductive tips are electrically connected by a conductive wire, whereinthe array of protruding tips is separated into a first electricallyconnected group and a second electrically connected group wherein one ofthe first or second electrically connected group is connected to a DCvoltage source that is positively biased and the other of the first orsecond electrically connected group is connected to a DC voltage sourcethat is negatively biased, and wherein the DC voltage source that ispositively biased and the DC voltage source that is negatively biasedare electrically connected to a functional controller that is capable ofalternating a supply of energy between the DC voltage source that isnegatively biased and the DC voltage source that is positively biased;passing a gas mixture comprising a reducing gas through the target area;energizing the rows of conductive tips by activating the DC voltagesource that is negatively biased to generate electrons within the targetarea, wherein at least a portion of the electrons attach to at least aportion of the reducing gas thereby forming a negatively chargedreducing gas; contacting the treating surface with the negativelycharged reducing gas to reduce the metal oxides on the treating surfaceof the substrate; and energizing the rows of conductive tips byactivating the DC voltage source that is positively biased to retrieveexcess electrons from the treating surface, wherein the rows ofconductive tips electrically connected to the DC voltage source that isnegatively biased and the rows of conductive tips electrically connectedto the DC voltage source that is positively biased are not energized atthe same time.
 2. The method of claim 1 wherein the reducing gas is agas selected from the group consisting of H₂, CO, SiH₄, Si₂H₆, CF₄, SF₆,CF₂Cl₂, HCl, BF₃, WF₆, UF₆, SiF₃, NF₃, CClF₃, HF, NH₃, H₂S, straight,branched or cyclic C₁ to C₁₀ hydrocarbons, formic acid, alcohols, acidicvapors having the following formula (III):

organic vapors having the following formula (IV):

and mixtures thereof, wherein substituents R in formula (III) andformula (IV) is an alkyl group, a substituted alkyl group, an arylgroup, or a substituted aryl group.
 3. The method of claim 2 wherein thereducing gas comprises H₂.
 4. The method of claim 3 wherein theconcentration of H₂ in the reducing gas is from 0.1 to 100 vol %.
 5. Themethod of claim 1 wherein the gas mixture further comprises a carriergas selected from the group consisting of: nitrogen, helium, neon, argonkrypton xenon, radon, and mixtures therefore.
 6. The method of claim 1wherein the conductive tips are spaced apart from each other by adistance of from about 2 mm to 10 mm.
 7. The method of claim 6 whereinthe conductive tips are spaced apart from each other by a distance offrom about 5 mm to about 8 mm.
 8. The method of claim 7 wherein theconductive tips are spaced apart from each other by a distance of about5 mm.
 9. The method of claim 1 wherein the base electrode and theenergizing electrode are spaced apart from one another by a distance offrom about 0.5 cm to about 5.0 cm.
 10. The method of claim 1 wherein thebase electrode and the energizing electrode are spaced apart from oneanother by a distance of 1.0 cm.
 11. The method of claim 2 wherein thevoltage ranges from 0.1 kV to 30 kV.
 12. The method of claim 2 whereinthe substrate is at a temperature ranging from 100° C. to 400° C. 13.The method of claim 1 wherein the voltage is pulsed at a frequencybetween 0 kHz and 50 kHz to prevent arcing.
 14. The method of claim 1wherein the insulated plate comprises a material selected from the groupconsisting of: quartz, a ceramic material, a polymer, and mixturesthereof.
 15. The method of claim 1 wherein the insulated plate is aquartz plate.
 16. The method of claim 1 wherein the insulated platecomprises a polymer.
 17. The method of claim 16 wherein the polymer isan epoxy polymer.
 18. The method of claim 1 wherein the treating surfacefurther comprises solder bumps.
 19. The method of claim 1 wherein thesubstrate is an insulated substrate selected from the group consistingof: a rigid epoxy glass laminate substrate, a flexible flex polymericsubstrate, a substrate used in an integrated circuit interconnectionscheme, a high density interconnect, a substrate used in stackedintegrated circuit, and a substrate used in a stacked package.
 20. Themethod of claim 1 wherein the protruding conductive tips are removablyattached to the insulated plate.
 21. The method of claim 13 wherein thefrequency is between 0 kHz and 30 kHz.